Patent application title: SILICON ON INSULATOR (SOI) WAFER AND PROCESS FOR PRODUCING SAME

Abstract:

In a manufacturing method for manufacturing a silicon on insulator (SOI)
wafer, an ion injection layer is formed within the wafer, by injecting a
hydrogen ion or a rare gas ion from a surface of the single crystal
silicon wafer, the ion injection surface of the single crystal silicon
wafer and/or a surface of the transparent insulation substrate is
processed using plasma and/or ozone, the ion injection surface of the
single crystal silicon wafer is bonded to the surface of the transparent
insulation substrate, by bringing them into close contact with each other
at room temperature, with the processed surface(s) as bonding surface(s),
and an SOI layer is formed on the transparent insulation substrate, by
mechanically peeling the single crystal silicon wafer by giving an impact
to the ion injection layer.

Claims:

1. A manufacturing method for manufacturing a silicon on insulator (SOI)
wafer by bonding a single crystal silicon wafer to a transparent
insulation substrate, and thereafter making the single crystal silicon
wafer to be thinned to form an SOI layer on the transparent insulation
substrate, the manufacturing method performing at least:forming an ion
injection layer within the single crystal silicon wafer, by injecting at
least one of a hydrogen ion and a rare gas ion from a surface of the
single crystal silicon wafer;processing the ion injection surface of the
single crystal silicon wafer and/or a surface of the transparent
insulation substrate using plasma and/or ozone;bonding the ion injection
surface of the single crystal silicon wafer to a surface of the
transparent insulation substrate, by bringing them into close contact
with each other at room temperature, with the processed surface(s) as
bonding surface(s); andforming an SOI layer on the transparent insulation
substrate, by mechanically peeling the single crystal silicon wafer by
giving an impact to the ion injection layer.

2. The manufacturing method as set forth in claim 1, further comprising
performing, after the bonding of the ion injection surface and before the
forming of an SOI layer:raising a bonding strength by performing thermal
processing to the bonded wafer under a temperature of 100-300.degree. C.

3. The manufacturing method as set forth in claim 1, wherein mirror
polishing is provided to a surface of the SOI layer of the SOI wafer
obtained in the forming of an SOI layer.

4. The manufacturing method as set forth in claim 1, wherein the
transparent insulation substrate is one of a silica substrate, a sapphire
(alumina) substrate, and a glass substrate.

5. The manufacturing method as set forth in claim 1, wherein an ion
injection dose used in the forming of an ion injection layer is greater
than 8.times.10.sup.16/cm.sup.2.

6. An SOI wafer manufactured according to the manufacturing method as set
forth in claim 1.

7. The manufacturing method as set forth in claim 1, further
comprising:forming an insulation film on a surface of the single crystal
silicon wafer in advance, prior to the forming of an ion injection layer.

8. The manufacturing method as set forth in claim 1, further
comprising:providing mirror polishing to a surface of the SOI layer,
after the forming of an SOI layer.

9. The manufacturing method as set forth in claim 1, wherein the obtaining
of an SOI wafer comprises blowing a jet of gas or liquid continuously or
discontinuously.

Description:

CROSS REFERENCE TO RELATED APPLICATION

[0001]The present application is continuation application of
PCT/JP2006/313910 filed on Jul. 12, 2006 which claims priority from
Japanese Patent Application No. 2005-374884 filed on Dec. 27, 2005, the
contents of both applications being incorporated herein by reference.

BACKGROUND

[0002]1. Technical Field

[0003]The present invention relates to a manufacturing method of an SOI
wafer and to an SOI wafer. Particularly, the present invention relates to
a manufacturing method of an SOI wafer in which an SOI layer is formed on
a transparent insulation substrate, and to the SOI wafer.

[0004]2. Description of the Related Art

[0005]An SOI wafer including an SOI (silicon on insulator) structure in
which a silicon single crystal layer is formed on an insulator is suited
for fabricating a semiconductor integrated circuit of a high density, and
is expected to be applied to optical devices such as "TFT-LCD" (thin film
transistor-liquid crystal display).

[0006]An SOI wafer in which an SOI layer is formed on a transparent silica
substrate is used as such an optical device, for example. In this case,
the substrate is a complete insulator, and so does not affect the
mobility of the carrier within the SOI layer. Consequently, the mobility
of the carrier in the SOI layer will be extremely high, thereby yielding
a noticeable effect particularly when driven in high frequency. In
addition, in such an SOI wafer, a driving circuit can be formed in an
integral manner in the periphery of the TFT region, which enables high
density mounting.

[0007]In such an SOI wafer for use as an optical device, the thickness of
the SOI layer should be as thin as about 0.5 μm, for example.
Accordingly, the bonding strength between the silica substrate and the
SOI layer should be sufficiently strong and firm to endure grinding,
polishing for making the SOI layer to be as thin as the level of the
stated thickness, and to withstand the thermal and mechanical stresses
exercised onto the SOI layer in manufacturing the device. To enhance the
bonding strength, it has been required to perform thermal processing
under a high temperature.

[0008]However, the thermal expansion coefficient differs between a silica
substrate and an SOI layer. This occasionally causes stress due to
thermal deformation during the thermal processing for bonding, during the
cooling processing after the bonding, or during the grinding or polishing
processing, thereby causing the silica substrate or the SOI layer to
crack, or to break due to flaking. Such a problem is not confined to a
case where the insulation transparent substrate is made of silica, and
may equally happen when bonding a single crystal silicon wafer to a
substrate having a different thermal expansion coefficient.

[0009]So as to solve the mentioned problem, a technology has been
disclosed for alleviating the effect of thermal stress incident to
thermal processing, by performing a thermal bonding processing process
and a thin film process alternately and step by step, in an SOI wafer
manufacturing method adopting a hydrogen ion injection peeling method
(e.g. Patent Document No. 1: Japanese Patent Application Publication No.
11-145438).

SUMMARY

[0010]Regarding a manufacturing method of an SOI wafer in which an SOI
layer is formed on a transparent insulation substrate, it is an object of
an aspect of the innovations herein to provide a manufacturing method of
an SOI wafer and to provide an SOI wafer, by which thermal deformation,
flaking, cracking, or the like attributable to the difference in thermal
expansion coefficient between a transparent insulation substrate and an
SOI layer is prevented with a simple process.

[0011]According to the first aspect related to the innovations herein, one
exemplary manufacturing method is a manufacturing method for
manufacturing an SOI wafer by bonding a single crystal silicon wafer to a
transparent insulation substrate, and thereafter making the single
crystal silicon wafer to be thinned to form an SOI layer on the
transparent insulation substrate, the manufacturing method characterized
in performing at least: a step of forming an ion injection layer within
the single crystal silicon wafer, by injecting at least one of a hydrogen
ion and a rare gas ion from a surface of the single crystal silicon
wafer; a step of processing the ion injection surface of the single
crystal silicon wafer and/or a surface of the transparent insulation
substrate using plasma and/or ozone; a step of bonding the ion injection
surface of the single crystal silicon wafer to the surface of the
transparent insulation substrate, by bringing them into close contact
with each other at room temperature, with the ion injection surface and
the surface as bonding surfaces; and a step of forming an SOI layer on
the transparent insulation substrate, by mechanically peeling the single
crystal silicon wafer by giving an impact to the ion injection layer.

[0012]By processing the ion injection surface of the single crystal
silicon wafer and/or the surface of the transparent insulation substrate
using plasma and/or ozone, an OH group will be increased and activated on
the injection surface of the wafer and/or a surface of the substrate. If
the ion injection surface of the single crystal silicon wafer and the
surface of the transparent insulation substrate, under such a state, are
brought into close contact with each other in a room temperature to be
bonded, with the processed surfaces as the bonding surfaces, the surfaces
brought into close contact will be firmly bonded by means of hydrogen
bonding, to obtain sufficiently firm bonding even without providing high
temperature thermal processing for raising the bonding strength in later
stages. In addition, since the bonding surfaces are firmly bonded to each
other in the above way, thereafter a thin SOI layer can be formed on the
transparent insulation substrate by mechanically peeling the single
crystal silicon wafer by giving an impact to the ion injection layer.
This means that a thin film can be obtained even without performing
thermal processing for peeling. This further indicates that an SOI wafer
can be manufactured without causing thermal deformation, flaking,
cracking, or the like attributable to the difference in thermal expansion
coefficient between the transparent insulation substrate and the single
crystal silicon wafer. In addition, since the hydrogen ion injection
peeling method is used, it is possible to manufacture an SOI wafer whose
SOI layer has a thin film thickness, a favorable film thickness evenness,
and excellent crystallization.

[0013]In this case, between the step of bonding and the step of forming an
SOI layer, it is preferable to perform a step of raising a bonding
strength by performing thermal processing to the bonded wafer under a
temperature of 100-300 degrees centigrade. As in the above way, if the
single crystal silicon wafer and the transparent insulation substrate
bonded to each other are subjected to the mechanical peeling step for
giving an impact to the ion injection layer, after raising the bonding
strength by performing thermal processing of a low temperature of 100-300
degrees centigrade which does not cause thermal deformation, it is
possible to manufacture an SOI wafer by more assuredly preventing the
generation of flaking, cracking, or the like of the bonding surfaces
attributable to the mechanical stress.

[0014]It is preferable that mirror polishing is provided to a surface of
the SOI layer of the SOI wafer obtained in the step of forming an SOI
layer.

[0015]In this way, by providing mirror polishing to a surface of the SOI
layer of the SOI wafer obtained in the step of forming an SOI layer, it
is possible to remove surface roughness of the SOI layer caused in the
peeling process or to remove the crystal defects or the like caused in
the ion injection process, thereby enabling to manufacture an SOI wafer
having an SOI layer whose surface is mirror polished and smooth.

[0016]It is preferable that the transparent insulation substrate is one of
a silica substrate, a sapphire (alumina) substrate, and a glass
substrate.

[0017]As in the above way, if the transparent insulation substrate is one
of a silica substrate, a sapphire (alumina) substrate, and a glass
substrate, it is possible to manufacture an SOI wafer suitable for
fabricating an optical device, since these substrates have a favorable
optical characteristic. Here, examples of the glass substrate include
highly clear opaque glass, borosilicate glass, alkali-free borosilicate
glass, aluminoborosilicate glass, and crystallized glass, in addition to
soda-lime glass in common use. When using a glass substrate that includes
alkali metal (e.g. soda-lime glass), it is desirable that a surface of
the glass substrate be provided with a diffusion protective film made of
spin-on glass, for preventing diffusion of alkali metal from the surface.

[0018]Further, it is preferable that an ion injection dose used in the
step of forming an ion injection layer is greater than 8×1016/cm2.

[0019]As in the above way, by setting the ion injection dose to be greater
than 8×1016/cm2 in forming the ion injection layer, the mechanical
peeling becomes easy.

[0020]In addition, provided according to the innovations herein is an SOI
wafer manufactured according to any of the manufacturing methods recited
above.

[0021]As in the above way, an SOI wafer manufactured according to any of
the above-described manufacturing methods has not caused any thermal
deformation, flaking, cracking, or the like during manufacturing, as well
as having a thinner film thickness and a more favorable film thickness
evenness, having excellent crystallization, and having an SOI layer on a
transparent insulation substrate having high carrier mobility, useful for
manufacturing various devices.

[0022]By adopting the manufacturing method of an SOI wafer according to
the innovations herein, the surfaces to be bonded are processed with
plasma and/or ozone prior to bonding of the single crystal silicon wafer
and the transparent insulation substrate, which increase and activates
the OH group on the surfaces. If the single crystal silicon wafer and the
transparent insulation substrate, under such a state, are brought into
close contact with each other in a room temperature to be bonded, the
surfaces brought to close contact will be sufficiently firm bonding even
without providing high temperature thermal processing for raising the
bonding strength in later stages. In addition, since the bonding surfaces
are firmly bonded to each other in the above way, thereafter a thin SOI
layer can be formed on the transparent insulation substrate by
mechanically peeling the single crystal silicon wafer by giving an impact
to the ion injection layer. This means that a thin film can be obtained
even without performing thermal processing for peeling. This further
indicates that an SOI wafer can be manufactured without causing thermal
deformation, flaking, cracking, or the like attributable to the
difference in thermal expansion coefficient between the transparent
insulation substrate and the single crystal silicon.

[0023]In addition, the SOI wafer according to the present invention is an
SOI wafer that does not cause thermal deformation, flaking, cracking, or
the like during manufacturing, has a thinner film thickness, a more
favorable evenness in film thickness, excellent crystallization, and an
SOI layer on a transparent insulation substrate having high carrier
mobility, useful for manufacturing various devices.

[0024]The summary clause does not necessarily describe all necessary
features of the embodiments of the present invention. The present
invention may also be a sub-combination of the features described above.
The above and other features and advantages of the present invention will
become more apparent from the following description of the embodiments
taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 is a process diagram showing one example of a manufacturing
method of an SOI wafer, according to the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0026]Some aspects of the invention will now be described based on the
embodiments, which do not intend to limit the scope of the present
invention, but exemplify the invention. All of the features and the
combinations thereof described in the embodiment are not necessarily
essential to the invention.

[0027]As mentioned above, regarding a manufacturing method of an SOI wafer
in which an SOI layer is formed on a transparent insulation substrate, so
as to prevent thermal deformation, flaking, cracking, or the like
attributable to the difference in thermal expansion coefficient between a
transparent insulation substrate and an SOI layer, a technology has been
already disclosed by which the effect of the thermal stress incident to
thermal processing is alleviated by performing a thermal bonding
processing process and a thin film process alternately and step by step,
in an SOI wafer manufacturing method adopting a hydrogen ion injection
peeling method. However for the purpose of improving the productivity of
the SOI wafer, a new technology has been nevertheless desired for solving
the stated problem by a smaller number of processes and in a short time.

[0028]In view of this, the inventors of the present invention have
conceived to enhance the bonding strength without performing the thermal
processing by preprocessing the surfaces to be bonded using plasma and/or
ozone processing, and to perform peeling mechanically instead of thermal
processing, thereby completing the present invention. As follows, some
aspects of the present invention are described by way of embodiments. The
present invention will not be limited to the following embodiments.

[0029]FIG. 1 is a process diagram showing one example of a manufacturing
method of an SOI wafer, according to the present invention.

[0030]First, a single crystal silicon wafer and a transparent insulation
substrate are prepared (process A).

[0031]The single crystal silicon wafer is not particularly limited as long
as it is obtained by slicing a single crystal grown by the Czochralski
method for example, which for example has a diameter of 100-300 mm, a
conductivity type of P-type or N-type, and a resistivity of about 10
Ωcm.

[0032]The transparent insulation substrate is not also particularly
limited. However if any of a silica substrate, a sapphire (alumina)
substrate, and a glass substrate, all of them having a favorable optical
characteristic, is used as a transparent insulation substrate, it is
possible to manufacture an SOI wafer suitable for fabricating an optical
device.

[0033]Next, at least one of a hydrogen ion and a rare gas ion is injected
from the surface of the single crystal silicon wafer, to form an ion
injection layer in the wafer (process B).

[0034]For example, at least one of a hydrogen ion and a rare gas ion in a
predetermined dose is injected from the surface of the single crystal
silicon wafer, with an injection energy capable of forming an ion
injection layer at the depth corresponding to a predetermined SOI layer
thickness (e.g. the depth of smaller than or equal to 0.5 μm), while
keeping the temperature of the single crystal silicon wafer to 250-450
degrees centigrade. An exemplary condition may be the injection energy of
20-100 keV and the injection dose of 1×1016-1×1017/cm2. Here,
so as to facilitate the peeling at the ion injection layer, the ion
injection dose should preferably be equal to or greater than
8×1016/cm2. In addition, if the ion injection is performed through
an insulation film such as a thin silicon oxide layer formed in advance
on a surface of the single crystal silicon wafer, an advantage of
restraining channeling of the injected ion will be obtained.

[0036]In adopting plasma processing, a single crystal silicon wafer and/or
a transparent insulation substrate, to which cleansing such as RCA
cleansing has been performed, are/is placed in a vacuum chamber, and a
gas for plasma processing (hereinafter simply "plasma gas") is
introduced. Then the single crystal silicon wafer and/or the transparent
insulation substrate are/is subjected to high frequency plasma of about
100 W for about 5-10 seconds, to perform plasma processing to the surface
thereof. In processing a single crystal silicon wafer, for oxidizing the
surface thereof, the plasma gas may be plasma of an oxygen gas. For not
oxidizing the surface of a single crystal silicon wafer, the plasma gas
may be a hydrogen gas, an argon gas, a mixture gas of them, or a mixture
gas of a hydrogen gas and a helium gas. Any gas is usable for processing
of a transparent insulation substrate.

[0037]In adopting ozone processing, a single crystal silicon wafer and/or
a transparent insulation substrate, to which cleansing such as RCA
cleansing has been performed, are/is placed in a chamber to which
atmospheric air is introduced, and a plasma gas such as a nitrogen gas,
an argon gas, or the like is introduced. Then the surfaces are treated
with ozone processing by generating high frequency plasma to convert the
oxygen in the atmospheric air into ozone. Here, it is possible to perform
any one of plasma processing and ozone processing, or it is also possible
to perform both of plasma processing and ozone processing.

[0038]By processing with plasma and/or ozone, the organic substances on
the surface of the single crystal silicon wafer and/or the transparent
insulation substrate are oxidized to be removed, and instead the OH group
on the surface is increased and activated. The surface to be processed
may be a bonding surface. For a single crystal silicon wafer, the surface
to be processed is an ion injection surface. The processing is desirably
performed to both of a single crystal silicon wafer and a transparent
insulation substrate. However the processing may be performed to only one
of the single crystal silicon wafer and the transparent insulation
substrate.

[0039]Then, the ion injection surface of the single crystal silicon wafer
and the surface of the transparent insulation substrate, to which plasma
processing and/or ozone processing are/is provided, are brought into
close contact with each other in a room temperature to be bonded, with
the ion injection surface and the surface as the bonding surfaces
(process D).

[0040]In the process C, at least one of the ion injection surface of the
single crystal silicon wafer and the surface of the transparent
insulation substrate is processed by plasma processing and/or ozone
processing. Consequently, the respective surfaces of the single crystal
silicon wafer and of the transparent insulation substrate are able to be
bonded to each other firmly, with a strength that can endure the
mechanical peeling in the later processes, by simply bringing them into
close contact with each other, under a reduced pressure or a normal
pressure, and in a temperature of about a general room temperature, for
example. This means that thermal bonding processing of equal to or more
than 1200° C. is not necessary, and so it is preferable since
there is no possibility of causing thermal deformation, flaking,
cracking, or the like attributable to the difference in thermal expansion
coefficient, which is a problem inherent in heating processes.

[0041]After this, the bonded wafer may be subjected to thermal processing
of a low temperature of 100-300° C., for enhancing the bonding
strength (process E).

[0042]For example, when the transparent insulation substrate is made of
silica, the thermal expansion coefficient is smaller than that of silicon
(i.e. Si: 2.33×10-6, and silica: 0.6×10-6). Therefore if
the silica transparent insulation substrate is heated after being bonded
to the silicon wafer having about the same thickness, the silicon wafer
will break when exceeding 300 degrees centigrade. Thermal processing of a
relatively low temperature as in this process E is desirable since it
does not have a possibility of causing thermal deformation, flaking,
cracking, or the like attributable to the difference in thermal expansion
coefficient. Note that in adopting a thermal processing furnace (i.e. a
batch processing type), a sufficient advantage is obtained if the thermal
processing time is about 0.5-24 hours.

[0043]Next, a single crystal silicon wafer is mechanically peeled by
giving an impact to the ion injection layer, to form an SOI layer on the
transparent insulation substrate (process F).

[0044]In the hydrogen ion injection peeling method, thermal processing is
performed to the bonded wafer in an inert gas atmosphere of about
500° C., to perform thermal peeling by means of a rearrangement
effect of crystal and an aggregating effect of air bubbles of injected
hydrogen. In contrast, the present invention performs mechanical peeling
by giving an impact to an ion injection layer, and so there is no
possibility of causing thermal deformation, flaking, cracking, or the
like that would happen incident to heating.

[0045]For giving an impact to the ion injection layer, a jet may be used
to blow a fluid such as gas, liquid, or the like continuously or
discontinuously from the side surface of the bonded wafer, for example.
However, another method may be adopted as long as the method causes
mechanical peeling by impact.

[0046]In the above way, an SOI wafer in which an SOI layer is formed on a
transparent insulation substrate is formed is obtained in the peeling
process. It is preferable to provide mirror polishing to a surface of the
SOI layer of the SOI wafer obtained in this way (process G). This mirror
polishing enables to remove surface roughness caused in the peeling
process (so-called "haze"), and to remove the crystal defects caused in
the vicinity of the SOI layer surface due to the ion injection. An
example of this mirror polishing is "touch polish" that removes an
extremely small thickness of 5-400 nm.

[0047]The SOI wafer produced by the processes of A-G has not caused any
thermal deformation, flaking, cracking, or the like, during
manufacturing, as well as having a thin film thickness, a favorable film
thickness evenness, excellent crystallization, and an SOI layer on a
transparent insulation substrate having high carrier mobility, useful for
manufacturing various devices.

[0048]Moreover, such an SOI wafer is particularly suited for fabrication
of an optical device such as a TFT-LCD, because of having an SOI layer on
the transparent insulation substrate.

Embodiment Example

[0049]A single crystal silicon wafer having a diameter of 200 mm and one
surface thereof being subjected to mirror polishing is prepared, as a
wafer for forming an SOI layer. A silicon oxide layer of 100 nm is formed
on the surface of the single crystal silicon wafer by thermal
oxidization. The surface roughness (Ra) of the oxide layer at the surface
subjected to mirror polishing (i.e. a surface to be bonded) was 0.2 nm.
The measurement was performed to the measurement region of 10
μm×10 μm using an atom force microscope.

[0050]As a transparent insulation substrate, a synthetic silica wafer
having a diameter of 200 mm and one surface thereof being subjected to
mirror polishing is prepared. The surface roughness (Ra) of the
transparent insulation substrate at the surface subjected to mirror
polishing (i.e. a surface to be bonded) was 0.19 nm. The apparatus and
the method of measuring have the same condition as the oxide layer of the
single crystal silicon wafer.

[0051]A hydrogen ion is selected as the ion to be injected to a single
crystal silicon wafer through the silicon oxide layer of 100 nm, and the
ion is injected under a condition of an injection energy of 35 keV and an
injection dose of 9×1016/cm2. The injection depth of the single
crystal silicon layer was 0.3 nm.

[0052]Next, the single crystal silicon wafer to which ion has been
injected is placed in a plasma processing apparatus, and an air is
introduced as a plasma gas. Then the high frequency plasma processing is
performed for 5-10 seconds by applying a high frequency of 13.56 MHz
under a reduced pressure condition of 2 Torr between parallel plate
electrodes having a diameter of 300 mm under a high frequency power of 50
W.

[0053]As for a synthetic silica wafer, the wafer is placed in a chamber to
which an atmospheric air is introduced, and an argon gas is introduced as
a plasma gas in narrow space between electrodes. Then by applying a high
frequency between the electrodes to generate plasma, the oxygen in the
atmospheric air becomes ozonized by the existence of the atmospheric air
between the plasma and the substrate. The surface to be bonded is
processed by means of the ozone. The processing time was set to 5-10
seconds.

[0054]The wafers to which surface processing was performed in the above
manner were brought into close contact at room temperature, to start
bonding by strongly pressing one end of the both wafers in the thickness
direction. Then after 48 hours in the room temperature, the bonding
surface was observed by human eyes. As a result, the bonding was
confirmed to extend throughout the substrate.

[0055]So as to confirm the bonding strength, one of the wafers is fixed,
and the wafer surface of the other wafer is provided with a stress in the
parallel direction, in an attempt to perform displacement in the lateral
direction, however the displacement did not occur.

[0056]Next, so as to peel the ion injection layer by giving an impact
thereto, blades of paper cutting scissors were placed at the side surface
of the bonded wafers in a diagonal position, thereby knocking in wedges
several times. Accordingly, the peeling was caused at the ion injection
layer, thereby obtaining an SOI wafer and a remaining single crystal
silicon wafer.

[0057]The SOI layer surface (peeling surface) was observed by human eyes.
As a result, the surface roughness was confirmed to be rougher than the
surface roughness of the attached surfaces (Ra=0.2 nm). Therefore
polishing is performed to remove the thickness of 100 nm, thereby
obtaining a smooth surface having surface roughness (Ra) of smaller than
or equal to 0.2 nm. The inside-surface film thickness evenness of this
SOI layer was also measured. As a result, favorable film thickness
evenness was confirmed, with the film thickness variation being
restrained to equal to or smaller than 10 nm within the wafer surface.
Furthermore, the crystallization of the SOI layer was evaluated by a
SECCO defect evaluation using a liquid resulting from diluting the SECCO
etching liquid according to a predetermined method. The confirmed defect
density was 2×103-6×103/cm2 which is a favorable value.

[0058]The above-described embodiments do not limit the invention. The
above-described embodiments are only illustrative, and includes a
configuration substantially the same as the technical concept recited in
the claims of the invention. Any configuration that has the same effects
or advantages is intended to be included in the technical concept of the
present invention.

[0059]For example, the SOI layer of the SOI wafer already subjected to the
processes A-F (or A-G) is already sufficiently thinned. Therefore the
high temperature thermal processing (at the temperature in the range
between equal to or greater than 500° C., and smaller than the
melting point of silicon) for further raising the bonding strength may be
optionally performed depending on purposes.